Retail stores have long made use of “electronic article surveillance” or EAS, which is used to detect shoplifters. EAS systems commonly in use today employ magnetic or electromagnetic fields to detect tags placed on items in the stores. There has been a recent movement toward using radio frequency identification (RFID) for retail EAS. When an item is paid for the tag on that item is deactivated or removed at the point of sale. The EAS system at the exit detects the presence of active tags as they leave the store, indicating the presence of an item which has not been purchased and is being stolen. When the EAS system detects a tag, an alarm is activated for the retailer's loss prevention team. The RFID tags can, advantageously also be used for tracking and inventory management in the “back room” of the retail establishment, in distribution centers, and even in the manufacturing environment. RFID tags have found many other applications as well, for example, tracking vehicles, tracking pets, and as entry credentials for secured environments.
In passive RFID systems the reader must transmit a radio frequency (RF) carrier signal so that tags can communicate to the reader by modulating their signals onto the reader's RF carrier. Tags modulate their signals onto the reader's RF carrier via inductive or capacitive coupling in the near field or via backscatter modulation in the far field. For some applications the reader must estimate the amount of RF carrier transmission which is being reflected into its own receiver in order to cancel the carrier. For example, most high performance readers use active carrier reflection cancelling to reduce the transmit power reflected into the reader's receiver. These active transmit reflection cancellers must use a sensor or other mechanism for determination of the reflected transmit power in order to adapt the active cancellation circuitry.